JP6595074B2 - Photocathode comprising a silicon substrate with a boron layer - Google Patents

Description

  This application is a US Provisional Application No. 61/679200 entitled “Photocathode With Low Noise And High Quantum Efficiency, High Spatial Resolution Low-Noise Image Sensor And Inspection Systems Incorporating an Image Sensor” filed Aug. 3, 2012. Claim priority.

  The present invention relates generally to low light sensitive detectors (sensors) used in connection with semiconductor wafer, reticle, or photomask inspection systems, and more particularly to photocathodes used in sensors for such inspection systems.

  The photocathode is a negatively chargeable electrode generally used in a photodetection element such as a photomultiplier tube, an image intensifier, and an electron impact type CCD (EBCCD). The photocathode includes a photosensitive compound that generates one (or a plurality of) electrons in response to each photon absorbed by the photoelectric effect when impacted by light quanta (photons). Photosensitive compounds used in modern photocathodes usually comprise an alkali metal. This is because the low work function of the alkali metal allows electrons to easily escape from the photocathode for detection by other structures of the host image sensor device. Compound semiconductors such as GaAs and InGaAs are also used to produce photocathodes for infrared sensitive devices in particular. Silicon photocathodes have been produced in the past, but silicon is efficient in capturing light, but only a small percentage of the generated electrons can escape from the silicon, reducing overall efficiency, making it noticeable No commercial use was seen.

  Photocathodes are generally divided into two large groups, conductive photocathodes and reflective photocathodes. Conductive photocathodes are typically formed on the surface of a window (eg, glass) that faces the light source being measured, and electrons exiting the photocathode pass through the output surface of the photocathode for detection ( That is, the electrons move away from the light source). A reflective photocathode is typically formed on an opaque metal electrode substrate through which light enters and electrons exit from the same “irradiated” surface. Reflective photocathodes simplify some of the tradeoffs between photocathode thickness and sensitivity described below, but are not suitable for use in imaging devices such as image intensifiers and EBCCD devices (but they are May be suitable for use in some photomultiplier tube configurations). Accordingly, in the following description, the term “photocathode” refers to a conductive photocathode unless otherwise noted.

  The photocathode is typically formed or mounted in a suitable host sensor housing (eg, a semiconductor or vacuum tube), and the sensor housing is such that the illuminated surface faces the target light source (ie, the photocathode is the host sensor). Positioned between the electronic measuring structure and the light source. When photons are absorbed by the photocathode, an average of about 50% of the generated electrons travel toward the irradiated side of the photocathode (ie, the side facing the light source through which the photon enters the photocathode). To do. The remaining 50% of the photoelectrons travel to the output surface of the photocathode and are emitted towards the sensor's electronic measurement structure if the photoelectrons have sufficient energy. As electrons are emitted from the output surface of the photocathode, they are typically accelerated toward the anode by an electric field in the host sensor and produce a corresponding measurable voltage or current that indicates the capture of one or more photons. Generate.

  A photomultiplier tube is a vacuum phototube that includes a photocathode, an anode, and a series of dynodes (electrodes), each dynode being continuously more positive than the previous dynode and higher than the last dynode. With potential anode. The photoelectrons emitted from the photocathode are accelerated by the photocathode dynode electric field and usually collide with the dynode, thereby releasing a plurality of secondary electrons accelerated by the subsequent dynode-to-dynode electric field. Nearly all of these secondary electrons collide with another dynode and generate additional electrons. Mostly, after a plurality of amplification steps by a plurality of dynodes, the electrons finally reach the anode. Thus, the photomultiplier tube generates a pulse of current (i.e., charge) each time a photon is absorbed, emitting photoelectrons in the appropriate direction. Since the generated charge is equal to the charge in a large number of electrons, if the gain is sufficiently high, it is possible to generate a charge that exceeds the noise level of electronics. Therefore, the photomultiplier tube can be a very sensitive detector for light in the ultraviolet, visible, and near infrared regions of the electromagnetic spectrum. These detectors amplify the current generated by incident light by 100 million times (ie, 160 dB) in multiple dynode stages, and individual photons are detected (for example) when the incident light flux is very low Make it possible.

  Image intensifiers are, for example, the intensity of light detected in an optical system to facilitate visual imaging of low light processes, or to convert invisible light sources such as near infrared or short wavelength infrared to visible This is another type of vacuum tube sensor device that uses phosphors to increase. In a typical image intensifier, the photoelectrons emitted from the photocathode collide with the phosphor with high energy (usually about 1 keV to about 20 keV), resulting in the phosphor generating a large number of photons. To accelerate toward the transparent anode covered with the phosphor. In some image intensifiers, a microchannel plate is placed between the photocathode and the phosphor to generate a plurality of secondary electrons from each photoelectron. Even without a microchannel plate, multiple photons can be generated at the output of the image intensifier for each absorbed photon. The emitted photons are guided to the image sensor by an optical member (such as a fiber optic bundle or lens). Since each absorbed photon can generate a large number of output photons, very low light levels are detected and measured, and under some conditions even a single photon is possible.

  The EBCCD is another sensor that operates similar to an image intensifier. Instead of a phosphor screen as an output, an image sensor such as a CCD is used to detect electrons emitted from the photocathode and accelerated by an electric field. In EBCCD, it is common to use a potential difference of about −2 kV or more to generate an electric field between the photocathode and the CCD, whereby the electrons emitted by the photocathode are accelerated and the CCD with high energy. A large number of electrons are generated inside the CCD. Since multiple electrons are generated for each photon detected, CCD readout and dark noise are less important than for direct photon detection. Compared to image intensifiers, EBCCD eliminates the cost of the optical components required to conduct light from the phosphor to the image sensor, and avoids image resolution degradation caused by those optical members.

  FIG. 11 shows a window 53, a photocathode 54 disposed on the inner surface of the window 53, and a charge coupled device disposed at the lower end of the housing 52 so that the photocathode 54 is separated from the CCD 55 by a vacuum gap 56. A conventional EBCCD 50 with a housing 52 including a (CCD) 55 is shown. By applying a voltage negative to the CCD voltage to the photocathode, an electric field is generated between the photocathode 54 and the CCD 55. Incoming photons 61 enter through window 53 and are absorbed by photocathode 54 to generate photoelectrons. If the photoelectrons 62 have sufficient energy to escape through the output side of the photocathode 54 (ie, downward in the drawing), the photoelectrons 62 enter the gap region 56. Since the CCD 55 is normally at a positive potential of 2 kV or higher with respect to the photocathode 54, the photoelectrons 62 accelerate toward the CCD 55 to reach an energy above about 2 keV, so that the photoelectrons generally cause a plurality of electrons to be transferred to the CCD 55. Generate internally. The electrons generated within the CCD 55 are then conducted (eg, by pins 57) to a processing system (not shown) that is configured to generate images or other data associated with the detected photoelectrons.

  Prior art photocathodes require a difficult trade-off between conflicting requirements related to photon absorption and photoelectron emission. A suitable photocathode should have a high probability of absorbing photons at the relevant wavelengths and a high probability of generating one (or more) photoelectrons from the absorbed photons. A suitable photocathode should also have a high probability that any photoelectrons generated by the absorbed photons will escape from the photocathode. A thick photocathode increases the probability that incident photons will be absorbed, but also increases the probability that the emitted photoelectrons will be recombined (ie lost) before they escape. Specifically, recombination usually occurs at defects or impurities in the material forming the photocathode, so that the photoelectrons encounter defects or impurities as the distance that the photoelectrons must travel through the photocathode material increases. The probability of recombination increases. The material must have a low work function because only photoelectrons with an energy close to or above the work function of the material have a reasonable probability of exiting the material.

  A typical photocathode is optimized for a relatively narrow range of wavelengths. For example, since the amount of light scattered from small particles generally depends more on the ratio of particle or defect size to wavelength than other factors, ultraviolet wavelengths are used to detect small particles and defects in semiconductor wafers. It is particularly useful in the semiconductor manufacturing industry. Most photocathode materials absorb ultraviolet light strongly. Photocathodes optimized with respect to UV wavelengths according to the prior art usually need to be thin because UV photons are absorbed near the illuminated surface. If the photocathode is not thin, the photoelectrons can be less likely to escape from the output surface of the photocathode. In general, only photoelectrons that escape at the side of the photocathode facing the phosphor or image detector will generate an output signal. Such thin photocathodes optimized for ultraviolet wavelengths generally have low sensitivity at visible and infrared wavelengths by allowing most of the incident photons at longer wavelengths to pass through the photocathode without being absorbed. become.

  Another limitation of prior art photocathodes is that the energy of the emitted photoelectrons varies with the wavelength of absorbed light and can be several eV if ultraviolet photons are absorbed. Since the direction in which the photoelectrons are emitted is random, this electron energy results in signal spreading in the horizontal direction. Furthermore, the diffusion varies with the wavelength of the absorbed photons, and becomes larger as the wavelength is shorter. In a thick photocathode, photoelectrons typically undergo multiple collisions before being emitted, and are likely to have an energy close to that determined by the temperature of the photocathode (i.e., the possibility of the electrons being heated). Is high). However, when electrons are subjected to multiple collisions within the photocathode, they are likely to recombine and be lost due to high levels of defects on and / or within the surface of the prior art photocathode material. Thus, a reduction in energy spread comes at the cost of a substantial reduction in sensitivity (most incident photons will no longer produce a signal).

  Single crystal (single crystal) silicon appears to overcome many of the shortcomings just described. Silicon absorbs all wavelengths shorter than about 1.1 μm. Silicon crystals can grow with very high purity and very few crystal defects. The electron recombination lifetime in high quality single crystal silicon has a long microsecond number and can be hundreds of microseconds in the highest quality material. Such a long recombination lifetime allows electrons generated many microns away from the surface to move to the surface with a low possibility of recombination.

  However, despite the many advantages, the development of commercial silicon-based photocathodes has been hampered by two main drawbacks.

  One disadvantage of silicon is that it has a relatively high work function (Allen and Gobelli, “Primarily clean silicon work function, photoelectron threshold, which prevents the emission of photoelectrons generated by photon absorption” And “Surface condition” (“Work Function, Photoelectric Threshold, and Surface States of Atomically Clean Silicon”), Physical Review 127, January 150, page 150-158, about 4.8 eV). is there. The work function of a material is the energy difference between an electron at the Fermi level and an electron at the vacuum level (ie, exiting the material). The relatively large band gap of silicon means that the heated electrons cannot escape from the silicon. Even ultraviolet photons absorbed near the surface of silicon do not generate much photocurrent because they do not have enough energy for the photoelectrons to escape. For example, a photon energy of 6.5 eV produces photoelectrons having an energy of about 3 eV (since direct absorption is more likely than indirect absorption at such wavelengths). Photoelectrons having an energy of about 3 eV cannot escape from silicon due to the work function of silicon.

A second and more serious problem of using silicon as the photocathode material is that silicon is very prone to generating native oxide on its surface. Even in the vacuum, native oxides will eventually form when small amounts of oxygen and moisture present in the vacuum react with the silicon surface. The interface between silicon and silicon dioxide has defects (due to dangling bonds) that are very likely to recombine electrons. Furthermore, the band gap of silicon dioxide is large (about 8 eV), and even when the oxide is very thin (the native oxide on a very smooth silicon surface is usually about 2 nm thin), the electrons escape. Brings further obstacles beyond the work function that must be overcome. The defect density in silicon relative to the oxide interface can be reduced by removing the native oxide and increasing the thermal oxide at a high temperature, for example, about 900-1000 ° C. Such a layer can be stable when increased from about 1.5 nm to a thickness of 2 nm. However, good thermal oxides have a significant defect density at the interface with silicon (usually 10 ⁹ to 10 ¹¹ defects per cm ² ) and are high in oxides bonded with a minimum thickness around 2 nm. The bandgap still presents a significant obstacle to the escape of electrons even though the work function can be overcome. A thin silicon nitride layer can be used to prevent an increase in the native oxide layer in the silicon, but the density of defects is higher in the silicon relative to the silicon nitride interface than in the silicon dioxide interface, and the band gap for silicon nitride (approximately 5eV) is large enough to prevent most electrons from escaping from the surface. For these reasons, silicon has not seen significant commercial use as a photocathode.

US Patent Application Publication No. 2006/0055321

  Therefore, what is needed is a photocathode that overcomes some or all of the limitations of the prior art.

  The present invention relates to a photocathode structure including a silicon substrate, a boron (first) layer formed on at least an output surface of the silicon substrate, and a low work function (second) layer formed on the boron layer. The silicon substrate is preferably essentially defect-free single crystal (single crystal) silicon having a thickness in the range of about 10 nm to about 100 μm, depending in part on the wavelength of light to be captured. The boron layer is on clean and smooth silicon (eg, about 600 ° C. to 800 ° C. to produce a pinhole free boron layer having a thickness in the range of 1-5 nm (preferably about 2 nm). It is desirable to be formed using a high temperature deposition process so that the boron layer avoids silicon oxidation problems by ensuring that the silicon surface is sealed against oxidation. A low work function material (eg, either an alkali metal such as cesium or an alkali metal oxide such as cesium oxide) is then applied over the boron layer to allow emission of electrons from the silicon substrate. The deposited, low work function material layer avoids the relatively high work function problem of silicon by efficiently generating negative electron affinity devices. Thus, by producing a photocathode having both a smooth boron layer and a low work function material layer formed on a single crystal silicon substrate, the present invention provides an extensive commercial application of silicon-based photocathodes. It provides the beneficial properties of silicon (ie, sub-1 μm wavelength absorption, high purity / low defect material, and long electron recombination time) while avoiding the downsides that prevented its use.

  According to various other embodiments of the present invention, various additional layers and structures are used to further enhance the beneficial properties of the creative photocathode structure. In some embodiments, a second boron layer (third layer) is formed on the irradiated (first) surface of the silicon substrate to further prevent oxides and defects that can reduce photon absorption. In order to further increase the absorption of photons, an antireflection material layer (fourth layer) is disposed on the third layer. In some embodiments, a metal frame or metal grid and a voltage source are used to generate an external potential difference between the illuminated surface and the output surface of the silicon substrate to move electrons preferentially toward the output surface. Is used. In yet another embodiment, boron (or another p-type dopant) is used to create a p-type dopant region to create a potential gradient that directs the electrons away from the irradiated silicon surface where electrons can be recombined and lost. It diffuses into the silicon substrate through the irradiated surface so as to form.

  According to another specific embodiment, the inventive photocathode structure of the present invention is incorporated into various sensor structures to provide sensors that exhibit excellent low light sensing performance. In addition to the photocathode (positioned adjacent to the light receiving surface of the sensor), these sensor structures have a sensing surface facing the output surface of the photocathode and separated from the low work function material layer by an intervening gap. Having a detection element (e.g., a CCD or CMOS image sensor) that functions to detect photoelectrons emitted through the output surface of the photocathode and generate an electrical signal indicative of the capture of the photoelectrons. In some sensor embodiments, the sensor structure is an electron impact charge coupled device (EBCCD) that may (or may not) have a window on top of the photocathode. In other embodiments of the invention, the sensor is an image intensifier that may (or may not) have a window on top of the photocathode. In yet another embodiment of the present invention, the sensor is a photomultiplier tube that may (or may not) have a window on top of the photocathode.

  In some sensor embodiments, a second boron layer is formed on the irradiated surface of the photocathode to prevent oxide formation on the irradiated surface, and the second boron layer is formed to increase photon capture efficiency. An antireflective material layer is provided thereon. In some of these embodiments, the antireflective material layer is disposed between the window and the photocathode, while in other embodiments, the antireflective material layer also functions as the light receiving surface of the sensor (ie, The sensor does not have a window on the illuminated surface of the photocathode), further increasing the photon capture efficiency by the sensor. In other sensor embodiments that include a window on the illuminated surface of the photocathode, an anti-reflective material layer is provided on the window to increase photon capture efficiency.

  In some embodiments of the present invention, a sensor comprising a photocathode of the present invention also includes a silicon-based sensing element having an additional boron layer on the light receiving surface (ie, the surface of the sensing element faces the photocathode). Including. For example, if the sensor is an electron impact CCD (EBCCD) and the detection element is a CCD (usually formed on a silicon substrate), the sensor captures the electrons by preventing the formation of a silicon dioxide layer on the light receiving surface of the CCD. To increase efficiency, a boron layer is formed directly on the light receiving surface of the CCD during manufacture. In other embodiments, the sensor includes a CMOS detector (instead of a CCD) and an additional boron layer is formed on the light receiving surface of the CMOS detector.

  In other embodiments of the invention, a sensor including an inventive photocathode is used in a wafer, reticle, or photomask inspection system. Specifically, an inventive system is described herein for detecting an illumination source (eg, a laser system) for sending light onto a sample / wafer, a photon passing through or reflected by the sample / wafer. One or more sensors (eg, photomultiplier tubes, image intensifiers, or EBCCDs) using any of the inventive photocathodes described in, and a sample (wafer, reticle, Or an associated optical system for directing light / photons from the sample to the sensor.

  The present invention is illustrated by way of example and not limitation in the accompanying drawings.

1 is a cross-sectional side view illustrating a photocathode formed on a silicon substrate according to a simplified embodiment of the present invention. FIG. FIG. 6 is a cross-sectional side view of a silicon photocathode according to another specific embodiment of the invention. FIG. 6 is a cross-sectional side view of a silicon photocathode according to another specific embodiment of the invention. FIG. 6 is a cross-sectional side view of a silicon photocathode according to another specific embodiment of the invention. FIG. 6 is an enlarged partial cutaway side view showing doping and additional configurations formed on a photocathode silicon substrate formed in accordance with another exemplary embodiment of the present invention. FIG. 2 is an energy diagram showing typical electron energy levels inside a photocathode silicon substrate formed in accordance with the present invention. FIG. 5 is a cross-sectional side view of an exemplary sensor including a photocathode according to another specific embodiment of the present invention. FIG. 5 is a cross-sectional side view of an exemplary sensor including a photocathode according to another specific embodiment of the present invention. FIG. 5 is a cross-sectional side view of an exemplary sensor including a photocathode according to another specific embodiment of the present invention. FIG. 6 is a simplified diagram illustrating an inspection system according to another embodiment of the present invention. FIG. 6 is a simplified diagram illustrating an inspection system according to a further embodiment of the present invention. FIG. 6 is a simplified diagram illustrating an inspection system according to a further embodiment of the present invention. FIG. 6 is a simplified diagram illustrating an inspection system according to a further embodiment of the present invention. FIG. 6 is a simplified diagram illustrating an inspection system according to a further embodiment of the present invention. FIG. 6 is a simplified diagram illustrating another inspection system in accordance with another embodiment of the present invention. FIG. 6 is a simplified diagram illustrating another inspection system in accordance with another embodiment of the present invention. FIG. 6 is a simplified diagram illustrating another inspection system in accordance with another embodiment of the present invention. It is a cutaway side view which shows the conventional sensor.

  The present invention relates to improvements in low light sensors for semiconductor inspection systems. The following description is presented to enable any person skilled in the art to make and use the invention as provided in the context of a particular application and its requirements. As used herein, for example, “top”, “bottom”, “up”, “down”, “up”, “up”, “low”, “down” and “down” Such orientation terms are intended to provide a relative position for illustrative purposes, and are not intended to indicate an absolute structural reference. Various modifications to the preferred embodiments will be apparent to those skilled in the art and the generic principles defined herein may be applied to other embodiments. Accordingly, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

  FIG. 1 illustrates in cross-section a photocathode 100 according to a generalized embodiment of the present invention. The photocathode 100 generally has a silicon substrate 101 having an upward irradiated (top / first) surface 102 and an opposite downward output (bottom / second) surface 103, and boron disposed at least on the output surface 103. It includes a (first) layer 104 and a low work function (second) layer 105 disposed on the lower surface of the boron layer 104. When correctly positioned, the photocathode 100 absorbs photons 110 that enter the silicon substrate 101 through the irradiated (upper) surface 102 and typically generates photoelectrons 112 that are emitted from the substrate 101 through the output surface 103. In that it operates in the same way as a conventional photocathode. However, FIG. 1 shows a hatched structure surrounding the illuminated surface 102 to indicate that a window and / or protective coating and / or anti-reflection coating is optionally present on the illuminated surface 102. Some of these are addressed in the various specific embodiments described below.

In accordance with aspects of the present invention, the silicon substrate 101 is preferably p-type single crystal silicon doped by doping at a level below about 10 ¹⁹ cm ⁻³ , ie, a resistivity of about 0.005 Ωcm or greater (ie, One crystal silicon). Higher dopant concentrations reduce minority carrier lifetime and diffusion distance, so if the silicon is very thin, for example silicon thinner than about 1 μm, a dopant concentration higher than about 10 ¹⁹ cm ⁻³ can be used, but silicon If is greater than about 1 μm, a dopant concentration of less than about 10 ¹⁹ cm ⁻³ may be suitable. In other embodiments, the silicon substrate 101 comprises polycrystalline silicon, ie, multiple crystalline silicon. Depending on the intended wavelength operating range of the photocathode, the silicon can be between about 10 nm and about 100 μm thick. Since the silicon substrate 101 exhibits a band gap of about 1.1 eV, light having a vacuum wavelength shorter than about 1.1 μm is absorbed. Since the 1.1 eV band gap of the silicon substrate 101 is indirect, the absorption of wavelengths in the red and infrared portions of the spectrum is weak. Since the silicon substrate 101 also has a direct band gap of about 3.5 eV, it strongly absorbs deep ultraviolet wavelengths. Depending on the intended use of the photocathode 100, the silicon substrate 101 has a thickness T1 in the range of about 20 nm to about 100 μm. For example, to facilitate a high probability of absorbing photons in the infrared portion of the spectrum, the silicon substrate 101 is formed having a thickness T1 of about 10 μm or several tens of μm. Alternatively, in order to absorb the ultraviolet wavelength, the silicon substrate 101 is formed to have a thickness T1 ranging from several tens of nm to about 100 nm. In a practical embodiment, the silicon substrate 101 has a thickness T1 of about 1 μm to absorb at least 85% of non-reflecting incident photons over a wavelength range from vacuum ultraviolet to about 670 nm near the red end of the visible spectrum. . When the silicon substrate 101 has a single crystal (single crystal) structure grown with a very low density of crystal defects and high purity using known techniques, the number of photoelectrons generated inside the silicon substrate 101 is several. Has a potential lifetime of ten or several hundred microseconds (μs). In addition, due to the single crystal structure, the photoelectrons lose most of their extra energy and are partially or mostly thermalized with a low probability of recombination.

According to another aspect of the present invention, the boron layer 104 comprises substantially pure boron disposed directly on the output surface 103 of the silicon substrate 101. As used herein, the phrase “directly” related to the boron-silicon interface is used for thin layers of SiB _x that may occur at the Si / B interface (ie, a small single layer). Rather, it is intended to mean that there is no persistent intervening layer (eg, an oxide or SiN _x layer) separating the output surface 103 and the boron layer 104. However, the phrase “directly” does not exclude the presence of oxide between silicon and a portion of boron. The boron layer 104 is formed of F.S. so that the boron forms a pinhole free coating having a thickness T2 in the range of about 1 nm to 5 nm, preferably about 2 to 3 nm. “Chemical Vapor Deposition of α-Boron Layers on Silicon for Controlled Nanometer-Deep p” by F. Sarubbi et al. + n Junction Formation ”) (Journal of Electronic Materials, Vol. 39, No. 2 (February 2010), 162-173, ISSN 0361-5235) Used to form on clean and smooth silicon at elevated temperatures (ie, temperatures above about 500 ° C, preferably between about 600 ° C and 800 ° C). It is important to remove all the native oxide from the silicon, for example by water washing and subsequent in-situ thermal purification, before depositing boron, as Salbi et al. Boron deposition at lower temperatures is possible, but the coating will not be uniform and coatings thicker than 2 nm will need to be verified to be pinhole free. The advantage of the boron layer 104 is that such a pinhole free coating prevents the formation of native oxide on the output surface 103 when applied to a clean silicon surface. The advantage of providing the boron layer 104 between the silicon substrate 101 and the low work function material layer 105 (eg, alkali metal or alkali metal oxide) is that boron is oxidized between the low work function material and the silicon dioxide. It is to prevent the formation of a silicon layer. As mentioned above, the silicon dioxide layer has a high band gap, and even a thin layer can prevent most of the electrons from leaving the silicon. Thus, the boron layer allows even low energy electrons to leave the silicon and enter the alkali metal or alkali metal oxide layer. Although it is well known in the art to coat a silicon photocathode with a low work function material such as cesium oxide, prior art devices can be used even when no oxide is present when the silicon layer is coated. The silicon dioxide interface between silicon and the low work function material could not be avoided. That is, in the absence of an impermeable pinhole free protective layer on silicon, oxygen will eventually migrate to the silicon surface and form an oxide layer. The advantage of using boron to form layer 104 is that even a thin pinhole free boron layer protects silicon without impregnating oxygen. Another advantage of the boron coating is that the density of interface traps and defects at the silicon-boron interface is generally lower than that at the silicon-silicon dioxide interface.

  In accordance with another aspect of the present invention, a low work function material layer 105 is provided to reduce the work function at the output surface 103 by creating negative electron affinity elements at the output surface 103. In one embodiment, the low work function material layer 105 comprises at least one of an alkali metal or an alkali metal oxide having a low work function that allows electrons to easily escape from the silicon substrate 101. In the embodiment of the present invention, the alkali metal or alkali metal oxide is coated on the upper end of the boron layer 103 (that is, the output side of the photocathode 100). In some embodiments, the alkali metal or alkali metal oxide is cesium or cesium oxide. In other embodiments, other alkali metals, other alkali metal oxides, various alkali metals or mixtures of alkali metal oxides are used. In some embodiments, other elements are added to the alkali metal (s) or alkali metal oxide (s). In a preferred embodiment, the alkali metal or alkali metal oxide layer 105 has a thickness T3 of less than about 2 nm. In some embodiments, layer 105 is less than about 1 nm thick. Cesium and cesium oxide layers have been used for decades to create negative electron affinity surfaces in semiconductor photocathodes. The latest description is B.B. S. It can be found in a report entitled “Study of Negative Electron Affinity GaAs Photocathodes” by B.S. Henderson.

  FIGS. 2 (A) to 2 (C) show inventive photocathodes according to various alternative embodiments of the present invention, in which additional features are added to further enhance the advantageous properties of the creative photocathode structure. Layers and structures are used. The exemplary embodiments illustrated are not intended to be exhaustive, and it will be appreciated that photocathodes are possible that include additional layer and structure combinations described below.

  FIG. 2A shows a photocathode 100A in cross-sectional view according to a first alternative embodiment. Similar to the structure described above, the photocathode 100 A includes a silicon substrate 101 having a boron layer 104 disposed on the output surface 103 and a low work function layer 105 disposed on the boron layer 104. Further, the photocathode 100A includes a second boron layer (third layer) 106 formed on the upward irradiated (first) surface 102 of the silicon substrate 101 using the technique described above with respect to the formation of the boron layer 103. Including. Boron absorbs at ultraviolet wavelengths, but by forming a second boron layer 106 having a thickness T4 of about 3 nm or less, only a fraction of the incident photons are absorbed. Further, since boron is a p-type dopant in silicon, the presence of the boron coating tends to pull photoelectrons away from the irradiated surface 102. Therefore, the efficiency of the photocathode 100A is improved by reducing the possibility of recombination of electrons on the irradiated surface 102. Similar results can be achieved by implanting p-type dopants in a very shallow layer at the surface, as described below in connection with FIG. In another embodiment of the invention (not shown), the irradiated surface 103 is not protected by a boron layer, but instead boron (or another p-type dopant) is implanted into the silicon below the irradiated surface 102. (Diffused). The presence of the oxide coating on the irradiated surface does not require a significant reduction in quantum efficiency, and by appropriately selecting the oxide thickness, the silicon reflectivity at the relevant wavelengths can be effectively reduced.

  2B, the silicon substrate 101, the lower boron layer 104 disposed on the output surface 103, the low work function layer 105 disposed on the boron layer 104, and the irradiated surface 102 are disposed. A photocathode 100B according to a second alternative embodiment including an upper boron layer 106 is shown in cross-section. The photocathode 100B differs from the above-described embodiment in that it further includes one or more antireflection coating layers (fourth layers) 107 disposed on the upper boron layer 106. The anti-reflective coating layer 107 is formed using materials that can be used for ultraviolet and deep ultraviolet anti-reflective layers, including but not limited to silicon dioxide, silicon nitride, magnesium fluoride, and hafnium dioxide. For photocathodes that require high quantum efficiency at ultraviolet wavelengths, ultraviolet photons are absorbed near the illuminated surface 102, so the density of trapped charges and defects at and near the illuminated surface 102 must be kept low. . As noted above, any dielectric material that directly covers the surface of the silicon substrate 101 creates a large portion of the dielectric material and defects and trapped charges at the surface, especially with respect to deep ultraviolet wavelengths that are absorbed near the surface. Will reduce the quantum efficiency. First, by forming the upper boron layer 106 as a thin (for example, about 2 nm to 5 nm) pinhole free layer, the irradiated surface 102 basically remains free of defects and trap charges, and the photocathode 100B , Including improved high quantum efficiency provided by the anti-reflective coating layer 107. The upper boron layer 106 can also at least partially protect the silicon substrate 101 from trapped charges in any additional layers disposed over the antireflective coating layer 107.

  2C, the silicon substrate 101, the lower boron layer 104 disposed on the output surface 103, the low work function layer 105 disposed on the boron layer 104, and the irradiated surface 102 are disposed. FIG. 6C shows a photocathode 100C according to a third alternative embodiment including an upper boron layer 106. FIG. In the photocathode 100C, the external voltage source 120 generates an external potential difference between the irradiated surface 102 and the output surface 103 in order to move electrons generated in the silicon substrate 101 preferentially toward the output surface 103. It is different from the above embodiment in that it is connected. In this embodiment, the output surface 103 is held at a positive potential with respect to the irradiated surface 102. Since the heavily doped silicon forming the substrate 101 is a weak conductor, an appropriate potential difference between the illuminated surface 102 and the output surface 103 (e.g., below about 5V) is generated by the output surface 103. To generate a fast flow of photoelectrons through the. In some embodiments, metal edges (not shown) or sparse metal grids 108-1 and 108 around the ends of the photocathode are used to ensure good electrical connection of the opposing faces of the photocathode 100C. -2 are formed on the upper boron layer 106 and the low work function layer 105, respectively.

  FIG. 3 shows a book with a dopant gradation intentionally spread from one or both of the illuminated surface 102 and the output surface 103 into the substrate 101 to direct photoelectrons in the silicon toward the output surface 103. FIG. 4 shows a cross-sectional view of a photocathode 100D according to another embodiment of the invention. The low work function material layer is omitted for the sake of brevity. In the illustrated embodiment (i.e., the embodiment where the irradiated surface 103 is covered by the upper boron layer 106), a thin boron silicide layer 126 is formed between silicon and boron. A part of boron spreads into the silicon so as to form a diffusion region 101-1 (indicated by the shaded region), thereby contributing to moving electrons away from the irradiated surface 102 toward the substrate 101. A gradient occurs. In these embodiments of the invention where the irradiated surface does not have a boron coating, a p-type dopant, such as boron, is implanted or diffused from the irradiated surface into the silicon to form a diffusion region 101-1. The Similarly, on the output surface 103, a very thin (one or a few single layer) boron silicide layer 134 is formed between the silicon substrate 101 and the boron layer 104, and a portion of the boron (shadowed) A short distance is diffused into the silicon so as to form a diffusion region 101-1). In some embodiments, the diffusion layer 101-2 may comprise other p-type or n-type dopants implanted into the silicon to alter the electric field gradient near the output surface 103.

  FIG. 4 is a diagram showing typical energy levels as a function of electron position in a cross-section of a typical inventive photocathode formed in accordance with the above-described embodiments. The vertical direction represents energy. However, this figure is not to scale and is distorted, and some aspects are emphasized to more clearly show important aspects of the photocathode. The irradiated (top) surface of the photocathode 410 is shown on the left side, and the output (bottom) surface 412 is shown on the right side. The dotted line 402 represents the Fermi level at the photocathode when no external voltage is applied to the photocathode. When no external voltage is applied, the Fermi level is basically the same for any part of the photocathode.

  Line 403 represents the top of the valence band in the semiconductor. The exposed surface 410 of the photocathode is significantly p-doped by either apparent doping or boron diffusion from the surface boron coating (not shown because it is only a few nanometers thick), or both. Has been. Due to the significant p-type doping near the surface, the Fermi level is just above the top of the valence band. For example, for high levels of boron doping, the gap between the Fermi level and the top of the valence band should be as small as about 0.045 eV. As the doping concentration decreases away from the surface, the gap between the Fermi level and the top of the valence band increases, thereby causing the conduction and valence bands to bend downward as they move away from the surface, as indicated by arrows 420.

  Line 404 represents the bottom of the conduction band. The difference between the bottom of the conduction band and the top of the valence band is called the band gap. In the case of silicon, the band gap is about 1.1 eV, but decreases when the dopant concentration is high. When free electrons are generated by photon absorption, they are in the conduction band. The electrons are initially generated with an energy approximately equal to the difference between the photon energy and the band gap. In silicon, excess energy is usually lost quickly, so electrons quickly reach energy close to the bottom of the conduction band. Since the downward slope indicated by the arrow 420 in the conduction band is close to the non-irradiated surface, any electrons generated near that surface will quickly leave the surface and reside at or near the irradiated surface 410. Less likely to recombine at any defect. Deep UV photons are very easily absorbed by the irradiated silicon surface 410 of a few nm, so this dopant profile near the surface allows high quantum efficiency of the photocathode at deep UV wavelengths.

  The photocathode second surface 412 is coated with a low work function material on a thin boron layer directly in contact with silicon, as described above. Since the low work function material is conductive, its Fermi level is in the conduction band. This is indicated by the solid line 425 as the junction of the Fermi level and the conduction band. Since both the boron layer and the low work function layer are only a few nm thick, they are shown as one combined conductive layer. As described above, a part of boron diffuses into the silicon and generates p-type silicon near the surface. In some embodiments, additional dopants can be included in the silicon. Electrons can reduce their energy by moving from low work function materials to p-type doped silicon. Thereby, a positive charge is generated on the surface 412. This positive charge causes the conduction and valence bands to bend downward as indicated at 422. The shape of the slope in the conduction and valence bands shown at 422 is such that both the dopant concentration profile away from the silicon surface 411 and into the silicon interior and the depletion region caused by the transfer of electrons from the low work function material into the silicon interior. By being present, it does not become monotonous. Depending on the precise shape of the dopant concentration profile, there may be small local minima or maxima in the conduction and valence band energy curves near the surface. Such small deviations from the monotonous shape can have a significant impact on device performance if the device height is only a few ten eV and / or any maximum width is only a few nanometers. Don't give.

  Dotted line 405 represents the vacuum energy level. The difference between 405 and 425 represents the work function of the low work function material in contact with the photocathode surface 412. In some preferred embodiments, the work function of the low work function material is low enough that the vacuum level 405 is below the energy level of the substantially flat region of the conduction band inside the silicon. The result is what is known as a negative electron affinity device. Electrons in the silicon conduction band can easily escape from the surface 412, resulting in an efficient photocathode. Even if the vacuum level 405 is several tenths of an eV above the substantially flat region of the conduction band inside the silicon, the potential for escaped electrons can still be very high. If the vacuum level 405 exceeds the substantially flat region of the conduction band inside the silicon, electrons can easily escape from the surface 412 if the surface 412 is slightly positive compared to the surface 410.

  Applying a positive voltage to the surface 412 with respect to the surface 410 results in a Fermi level slope down from left to right, with a similar slope to the slope inherent in the conduction and valence bands. Will be added. Thereby, electrons moving from surface 410 towards surface 412 will be able to accelerate and reach surface 412 with sufficient energy to have a high probability of exiting.

  In prior art photocathodes based on silicon, there will be a thin oxide layer on the silicon surface 411. This oxide is only about 2 nm thick, but represents a substantial obstacle to any electrons that try to escape. The band gap of silicon dioxide is about 8 eV. Such a large band gap results in a local peak in the conduction band that is several eV higher than the conduction band inside silicon. The boron layer on the surface 411 prevents oxygen or moisture from reaching the silicon surface and prevents an increase in the oxide layer, thus enabling an efficient photocathode.

  5 (A) to 5 (C) are simplified cross-sectional views illustrating various sensor structures in accordance with another specific embodiment, each of the sensor embodiments being at least one of the above-described embodiments. Including a creative photocathode structure according to one provides a sensor with excellent low light sensing performance that can be used, for example, in semiconductor inspection systems. Although the simplified sensor structure shown in FIGS. 5A-5C is consistent with an EBCCD type sensor, the illustrated sensor structure may be other sensor structures (eg, image intensifier sensors or photomultiplier tubes). It can be seen that the present invention can be similarly applied to a sensor.

  FIG. 5A shows a cross-sectional view of a sensor 200A according to the first sensor embodiment of the present invention. The sensor 200A generally has a low work function material for the photocathode 100 due to the gap region 206 in which the silicon photocathode 100, the detection element 210 (eg, CCD or CMOS image sensor), and the detection surface 212 of the detection element 210 are interposed. A housing 210 is operatively connected between the photocathode 100 and the detection element 210 so as to be separated from the (second) layer 105. The photocathode 100 is disposed adjacent to the light receiving surface 203A of the sensor 200A and is disposed so that the irradiated surface 102 faces outward from the detection element 210, whereby the photocathode 100 emits light (for example, photon 110). ) And orient the photocathode 100 to emit photoelectrons 112 to the sensing element 210 through the intervening gap region 206. As described above in connection with FIG. 1, the photocathode 100 includes a boron (first) layer 104 formed on the output (second) surface 103 of a (eg, single crystal) silicon substrate 101, and a boron layer. Characterized by having a low work function material layer 105 disposed on 104. As with most CCD and CMOS image sensor elements, the detection element 210 is electrically sensitive (eg, by way of one or more output pins 217) depending on the sensing structure for detecting the photoelectrons 112 and the detected photoelectrons. A circuit for generating the signal S.

  According to aspects of the illustrated embodiment, the photocathode 100 is aligned with the sidewalls and other portions of the housing 202A with respect to the emptied envelope (ie, the gap region 206 that is primarily filled with vacuum). It is bonded or hermetically sealed to a non-conductive or high-resistance glass or ceramic window 204A. In one particular embodiment, the bond between the window 204A and the photocathode 100 is formed by a silicon dioxide layer disposed around the end of the photocathode 100. In some embodiments, the silicon substrate 101 of the photocathode 100 can be tens to hundreds of microns thick. Such a thickness is strong enough to withstand the force due to atmospheric pressure even if there is no window on the top surface of the photocathode. Suitable materials for use in forming the window 204A include fused quartz, quartz, alumina (sapphire), magnesium fluoride, and calcium fluoride.

  According to another aspect of the first sensor embodiment, when a suitable voltage source 220 is operatively connected to the sensor 200A, electrons 112 emitted from the photocathode into the gap region 206 toward the sensing element 210 are generated. Sensor 200 is operatively disposed adjacent to or on photocathode 100 and detection element 210 such that an electric field that acts to accelerate is generated between photocathode 100 and detection element 210. A conductive structure (eg, similar to the grid structure described above in connection with FIG. 2C). In some embodiments, as described above in connection with FIG. 2C, the output surface 103 of the photocathode 100 is approximately about the illuminated surface 102 by a second voltage source (not shown). It is maintained at a positive potential less than 5V. When the light emission (photon) 110 is absorbed, electrons are emitted by the photocathode 100, and the photocathode 110 is maintained at a negative potential with respect to the detection element 210 by the voltage source 220, and thus is emitted into the gap region 206. The electrons 112 accelerate toward the detection element 210. In a preferred embodiment, the potential difference produced by voltage source 220 is in the range of about 100V to about 1500V.

  FIG. 5B shows the photocathode 100B, the detection element 210B (for example, a CCD or CMOS image sensor) formed on the (second) silicon substrate 211C, and the photocathode 100B at a certain distance from the detection element 210B. FIG. 6B shows a sensor 200B according to a second sensor embodiment that includes a housing 202 to hold. As described above with reference to FIG. 2B, the photocathode 100 (B) includes the second boron layer (third layer) 106 disposed directly on the irradiated (first) surface 102 of the silicon substrate 101. And an anti-reflective material (fourth) layer 107 disposed on the second boron layer 106. In this embodiment, since the housing 202B does not include a window disposed on the photocathode 100, the light receiving surface 203B of the sensor 200B is formed by the output (upper) surface of the antireflection material (fourth) layer 107. Is done. Thus, sensor 200B has the advantage of being more sensitive (ie, capable of detecting a smaller amount of light) than a sensor with a window because there is no loss due to reflection or absorption by the window. Therefore, the sensor 200B exhibits a relatively high sensitivity from the near infrared wavelength to the X-ray wavelength.

  FIG. 5C illustrates a photocathode 100 and a silicon-based sensing element 210C (eg, (second) silicon substrate 211C) that is secured to the housing 202C in a manner similar to that described above with respect to FIG. 5A. FIG. 7 shows a sensor 200C according to a third sensor embodiment including a CCD or CMOS image sensor formed thereon.

  According to aspects of the third sensor embodiment, the housing 202C includes an upper window portion 204C disposed over the photocathode 100 and a reflection formed on the window 204C to improve photon capture by the sensor 200C. And a prevention material layer 207C. In another embodiment, an additional anti-reflective material layer (not shown) is disposed between the photocathode 100 and the window 204C (ie, the photocathode 100 is, for example, the photocathode described above with respect to FIG. 2B). 100B).

  According to another aspect of the third sensor embodiment, in order to allow the electrons emitted from the photocathode 100 to be efficiently absorbed by the image sensor 210C, the (third) boron coating layer 214C is It is formed directly on the detection (upper) surface 212 of the image sensor 210C using the technique described above with respect to the photocathode 100. In a preferred embodiment, the gap distance G between the photocathode 100 and the image sensor 120 is between about 100 μm and about 1 mm. Since the boron coating layer 214C improves the efficiency of the image sensor 210C for low energy electrons, lower acceleration voltages and smaller gaps than standard in devices according to the prior art may be used. The advantage of a low acceleration voltage and small gap is that the spatial resolution of the sensor is improved and the response time is reduced (ie, the maximum operating frequency is increased). Photoelectron thermalization at the silicon photocathode also improves the spatial resolution of the image sensor.

  In other embodiments of the invention, the wafer, reticle or photomask inspection system passes through the sample / wafer with an illumination source (eg, a laser system) for sending light (photons) onto the sample / wafer, or A sensor (eg, photomultiplier tube, image intensifier or EBCCD) that uses any of the inventive photocathodes described above to detect photons reflected by them, and a sample (wafer, Reticle or photomask) and associated optics for directing light / photons from the sample to the sensor. Examples of those embodiments are shown in FIGS.

  FIG. 6 shows the main components of an inspection system 300A having a dark field and bright field inspection mode. The optical member used by system 300A includes a high numerical aperture large field objective lens 128, a tube lens 139 for setting or adjusting magnification, and a detector including a photocathode configured according to any of the embodiments described above. 200. When operating in dark field mode, the detector 200 includes an inventive photocathode in an EBCCD or image intensifier configuration similar to that shown in any of FIGS. 5 (A) to 5 (C). Further details in other aspects of this inspection system can be found in US Pat. No. 7,345,825, which is hereby incorporated by reference in its entirety.

  FIGS. 7A-7D illustrate aspects of a dark field inspection system that includes an inventive photocathode according to another exemplary embodiment of the present invention. In FIG. 7A, an inspection system 300B-1 irradiates a straight line 18 with light 14 passing through an optical member 11 comprising a lens or mirror 12 on the surface 20 of the wafer or photomask (sample) being inspected. To do. The condensing optical member 21 directs scattered light from the straight line to the sensor 200 using lenses and / or mirrors such as 22a and 22b. The optical axis 28 of the condensing optical member is not on the irradiation surface of the straight line 18. In some embodiments, axis 28 is generally perpendicular to line 18. Sensor 200 is an array sensor, such as a linear array sensor, including an inventive photocathode according to the embodiment shown in any of FIGS. 5 (A), 5 (B) and 5 (C), for example. 7 (B), 7 (C) and 7 (D) show multiple dark field collections including a detector 200 with a creative photocathode in combination with, for example, linear illumination as shown in FIG. 7 (A). 2 shows another configuration of the optical system (300B-2, 300B-3, and 300B-4, respectively). Further details of these inspection systems can be found in US Pat. No. 7,752,649, which is hereby incorporated by reference in its entirety. US Pat. No. 6,608,676, which is hereby incorporated by reference in its entirety, also describes a linear illumination system suitable for inspecting patterned or unpatterned wafers.

  FIG. 8 illustrates an unpatterned wafer inspection system 300C that includes an inventive photocathode in accordance with another exemplary embodiment of the present invention. Light from laser 1014 is directed to wafer 1012 by polarizing optical member 1016, beamforming optical member 1020, and reflecting mirrors such as 1022 and 1024, for example. Light scattered from the wafer is collected by mirrors and lenses such as 1038 and 1032 and sent to detectors 200-1 and 200-2 including photocathodes configured according to any of the embodiments described above. It is done. In some embodiments, detectors 200-1 and 200-2 comprise a photomultiplier tube that includes an inventive photocathode. Further details on non-patterned wafer inspection systems can be found in US Pat. No. 6,271,916, which is hereby incorporated by reference in its entirety.

  FIG. 9 shows a flooded wafer inspection system 300D according to another alternative embodiment of the present invention. The area of the wafer (sample) is illuminated by an off-axis light source. Light scattered from the wafer is collected by a condenser objective lens, passes through one or more apertures, splitters, and polarizers, and then a photocathode configured in accordance with any of the embodiments described above. To one or more image sensors 200-1 and 200-2. In some embodiments, image sensors 200-1 and 200-2 comprise an EBCCD or image intensifier that includes an inventive photocathode. Further details of this inspection system can be found in the co-pending US patent application 2013/0016346 by Romanovsky et al. Entitled “Wafer Inspection”, which is incorporated herein by reference in its entirety. Can be seen in In these inspection system embodiments, it is desirable that the wafer continue to move during inspection. The image sensor used in this embodiment of the present invention is an “inspection system using an electron impact charge coupled device and an EBCCD detector”, which is incorporated herein by reference in its entirety. Any of the techniques described in sharing co-pending US patent application 2013/0148112 entitled Systems Using EBCCD Detectors ") may be advantageously included.

  FIG. 10 illustrates a wafer inspection system 300E according to another alternative embodiment of the present invention. The inspection system 300E collects scattered light and directs the light to a plurality of sensors 200-1 to 200-5 via various apertures and polarizing beam splitters, according to any of the embodiments described above. Illumination subsystem 110E that provides tilted illumination and substantially normal illumination by the illustrated optical system including a typical photocathode.

  Prior art image intensifiers and electron impact CCDs have to compromise between sensitivity and spectral bandwidth. At best, good sensitivity is only possible for a narrow range of wavelengths. The present invention enables high sensitivity over a wider range of wavelengths by allowing the use of silicon as a photocathode. Furthermore, due to the high efficiency and low work function of the inventive photocathode, image intensifiers, photomultiplier tubes, and electron impact CCDs can operate at lower acceleration voltages in some embodiments. Thereby extending the lifetime of the device and improving the maximum operating frequency and / or spatial resolution.

  Prior art silicon photocathodes have an oxide layer on each side that prevents photoelectrons from escaping, resulting in low efficiency. Forming a boron layer on the output surface of silicon allows electrons to escape more easily, resulting in high efficiency.

  Image sensors that combine an inventive photocathode with a boron-coated CCD or CMOS image sensor exhibit higher quantum efficiency in a photocathode combined with the improved sensitivity of a boron-coated CCD.

  Dark field inspection systems that include detectors with creative photocathodes have a combination of high efficiency, very low noise levels, and high speed operation that cannot be achieved with conventional image and light sensors.

  Although the invention has been described with respect to certain specific embodiments, the inventive features of the invention are equally applicable to other embodiments, all of which are intended to fall within the scope of the invention. It will be clearly understood by those skilled in the art.

Claims (20)

A silicon substrate having opposing first and second surfaces;
A first layer mainly composed of boron disposed directly on the second surface of the silicon substrate;
A conductive photocathode comprising:
The photons entering the silicon substrate from the first surface are absorbed, photoelectrons are generated according to the absorbed photons, and the photoelectrons are emitted through the second surface and the first layer. Conducted photocathode.   The conductive photocathode according to claim 1, wherein the thickness of the silicon substrate is in the range of 10 μm to 100 μm.   The thickness of the silicon substrate and the dopant concentration of the silicon substrate are optimized to absorb photons having at least one of ultraviolet (UV) wavelength, far ultraviolet wavelength, and visible light wavelength. Conductive photocathode as described.   The conductive photocathode of claim 1, wherein the first layer has a thickness in the range of about 1 nm to 5 nm.   The conductive photocathode of claim 1, further comprising a second layer composed primarily of boron disposed directly on the first surface of the silicon substrate.   The conductive photocathode of claim 5, further comprising a third layer comprising an antireflective material disposed on the second layer.   The conductive photocathode of claim 4, further comprising means for generating an external potential difference between the first surface and the second surface of the silicon substrate.   P that spreads from the first surface toward the second surface such that the silicon substrate has a higher p-type dopant concentration in a portion near the first surface than in a portion disposed away from the first surface. The conductive photocathode of claim 1 comprising a mold diffusion region. A silicon substrate having an opposite irradiated surface and an output surface;
A first layer mainly composed of boron disposed directly on the output surface of the silicon substrate;
A conductive photocathode comprising:
The irradiated surface of the silicon substrate and the output are generated such that the silicon substrate generates photoelectrons according to photons entering the silicon substrate from the irradiated surface, and the generated photoelectrons are emitted from the output surface. The thickness between the surface and the dopant concentration of the silicon substrate was configured,
Conductive photocathode.   The conductive photocathode according to claim 9, wherein the thickness of the silicon substrate is in the range of 10 μm to 100 μm.   The thickness of the silicon substrate and the dopant concentration of the silicon substrate are optimized to absorb photons having at least one of an ultraviolet (UV) wavelength, a far ultraviolet wavelength, or a visible light wavelength. 9. The conductive photocathode according to 9.   The conductive photocathode of claim 9, wherein the first layer has a thickness in the range of about 1 nm to 5 nm.   The conductive photocathode according to claim 9, further comprising a second layer mainly made of boron disposed directly on the irradiated surface of the silicon substrate.   The conductive photocathode of claim 13, further comprising a third layer comprising an antireflective material disposed on the second layer.   The conductive photocathode according to claim 13, further comprising means for generating an external potential difference between the irradiated surface and the output surface of the silicon substrate.   A p-type that spreads from the irradiated surface toward the output surface so that the silicon substrate has a higher p-type dopant concentration in a portion near the irradiated surface than in a portion arranged away from the irradiated surface. The conductive photocathode of claim 9, comprising a diffusion region. A silicon substrate having opposing first and second surfaces;
A first layer mainly composed of boron disposed directly on the second surface of the silicon substrate;
A second layer mainly composed of boron disposed directly on the first surface of the silicon substrate;
A conductive photocathode comprising:
A conductive photocathode, wherein the first surface and the second surface include a pinhole free boron layer each having a thickness in the range of 1 nm to 5 nm.   The thickness of the silicon substrate and the dopant concentration of the silicon substrate are optimized to absorb photons that pass through the second layer and the first surface and enter the silicon substrate, and photoelectrons according to the absorbed photons 18. The conductive photocathode of claim 17, wherein the photoelectrons are emitted from the second surface and the first layer.   The conductive photocathode according to claim 18, wherein the thickness of the silicon substrate is in the range of 10 μm to 100 μm.   The conductive photocathode of claim 18, wherein the thickness and the dopant concentration are optimized to absorb photons having at least one of an ultraviolet (UV) wavelength, a far ultraviolet wavelength, or a visible light wavelength.